Mahogany Tide Just Wouldn’t Go Away!

As of early July 14, much of the Severn River was still experiencing the muddy-orange-brown color caused by the algae bloom known as the Mahogany Tide.

The ugly, rusty, orange-brown color is caused by an algae species called prorocentrum minimum.

Algae blooms are natural occurrence and are a food source for fish.

But when you mix an overload of nutrients (nitrogen and phosphorus) with warming waters and sunlight, the prorocentrum minimum explode out of control and take over the river and specially the creeks.

You could easily see thick swarms of the algae swimming in “schools” through the water and giving it a gritty appearance.

At times, there were clear delineations between algae bloom and clear water (see pic at left).

Maryland’s Department of Natural Resources classifies this as a harmful algae bloom (HAB). Click here for DNR’s HAB webpage.

Fortunately, the Mahogany Bloom type of algae is not usually harmful to humans, so swimming seems OK during a bloom – if you overlook the color.
This algae bloom creates two problems.

First, it’s so thick it blocks out sunlight that our underwater grasses need to thrive. This could diminish the amount of summer grasses in 2020.  Second, as the algae die off, their decomposition then depletes the oxygen from the water, creating dead zones and fish kills. this is already happening.

To help track the Mahogany Bloom, our newly formed Algae Loggers team filed reports on local conditions.

To become an Algae Logger to help track the next bloom, send an email to: and put Algae in the message box.

— Mackenzie Miller